actually, you get the same problem after the flood story, when at best you get an awful lot of first-cousin pairings, and very little genetic diversity, which... is not good genetically speaking. Whether option "2" is available to you depends a bit on your particular brand of Christianity; there are a spectrum of views on that, from "it is divine revelation as fact", through to "just 'it was Me' is the key point"
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Marc Gravell♦Dec 22 '11 at 8:55

2

re: I highly doubt that our species began with many acts of incest (not just for religious/legal/moral reasons but biological reasons as well), so how did humanity continue? Regardless of one's understanding of biological origins, I would think incest would be pretty much a given amonst those first generation(s) of humans.
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StevenDec 22 '11 at 19:48

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I've heard it said that their gene pools were so "rich" that they were able to (eventually) produce all the different races we have today. I have no idea how credible that idea is though...
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BenAug 24 '12 at 15:10

3 Answers
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The prohibition on incest didn't come about until the covenant in Leviticus & Deuteronomy, and to accuse Cain, Abel, and Seth of incest is to accuse them ex post facto.

As an aside, a Young Earth Creationist would date the Creation to 4004 BC, and the Exodus (and hence the Covenant) to about 1440BC. As such, you would be accusing them of a crime that wasn't mandated for nearly 2600 years.

If you want to say, "but surely the law goes back further than the covenant," you get into territory of, "So when did it become a law?" Incest is prohibited in most secular cultures today because it weakens the gene pool. Over time, the level of closeness has gotten wider in order to ensure a wider mixing. In modern times, a first or second cousin is off-limits. If I remember correctly in Leviticus, immediate family was prohibited, but I believe that cousins were ok. If you follow the trajectory backwards, you'd probably arrive at a date at which it was ok even for family members to procreate together, societally speaking.

An objection has been raised worth noting:

regardless, biology hasn't changed. legally incest wasn't "wrong" yet but humanity does not tend to survive well on inbreeding.

My response stands, however:

Agreed that humanity doesn't tend to survive well on inbreeding, but it fares even more poorly if the first of the species refrain from procreation altogether

"off limits" is probably over-stating it; "discouraged", certainly, but generally legal (but varies with local law). For example, it is legal in the UK, although not particularly common (it is much more common in some immigrant communities that have a previous cultural bias towards cousin marriage)
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Marc Gravell♦Dec 22 '11 at 8:59

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Even if you interpret Nephilim to be non-human creatures that can cross-breed (which is debated) trying to apply that possibility as a solution to this problem creates far more problems than you started with and causes God's judgement to be inconsistent.
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Caleb♦Dec 22 '11 at 10:16

The reason why incest is medically bad is because it makes genetic birth defects more likely. Most genetic defects are recessive. If two people who both have the gene have children, the childrenm may inherit the gene from both parents, and it will materialize. But if someone with the bad gene marries someone without the gene, then their children have one good gene and one bad gene, and -- assuming it's receissive -- they will not manifest the defect.

Bad genes are mutations, that is, DNA damaged by toxic chemicals, radiation, etc. Adam and Eve were presumably created with no bad genes. It took time for mutations to accumulate. So incest was not a medical problem for the first generations.

Incest also has social implications, the confusion of roles of "sister" and "wife" or "brother" and "husband". That would have been less of an issue before the Flood, when people lived longer. If Eve lived to be circa 900 years old like Adam, and if she was fertile for the same percentage of her life as a modern woman. That is, if a modern woman is fertile from mid teens to mid 40's, circa 30 years or 1/2 to 1/3 of her life, then if Eve lived to be 900, she might have been fertile for 300 years. She could have had children decades or centuries apart, who would not have been raised together.

And by the way, if Eve was fertile for several hundred years, she could have had dozens or even a hundred or more children in her life. And none of them ever called.

Maybe it's an allegory and not intended to be taken as literal as we often do.

I've always thought that there were 'others' with whom Cain mated. Within the known history of the world outside of the Bible, I speculate that the first humans were Adam and Eve. That is, they were the first intelligent beings who's children mated with the Neanderthals.

It is scientifically speculated that Homo Sapiens came out of Africa where they encountered the Neanderthals, who were stronger and better hunters (implying warriors) than the Homo Sapiens. The Homo Sapiens retreated back into Africa for years and then came back north, but this time for some unknown reason (I believe that it's the fruit of knowledge of good and evil) this time they had an enlarged brain. The rest as they say is history.

FYI, Walter Brueggemann would be your reference here. He says "There is no way you can over-allegorize the first 12 chapters of Genesis." Personally, I do think these things actually happened, so, I'm not going down that road, but if you'd like to support your answer a bit more, his commentary of Genesis would be the best place to start.
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Affable GeekDec 22 '11 at 15:17

RE horns on Moses: I don't know any Biblical literalist who says we should take TRANSLATIONS as inerrant. We say that the ORIGINAL MANUSCRIPTS were inerrant. My faith in the authority of the word of God is not shaken because some printer made a typo, or some cultist misquotes scripture.
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JayFeb 21 '14 at 16:45